1. Field of the Invention
This invention relates to grafted propylene copolymer products and adhesive blends containing same. More particularly, the invention relates to impact propylene-ethylene copolymer grafted with carboxylic acids and derivatives thereof having high graft concentrations at significantly lower melt flow rates (MFRs).
2. Description of the Prior Art
Propylene-based polymers grafted with ethylenically unsaturated carboxylic acids or derivatives are well-known and widely used as adhesive blends for adhesion to one or more polypropylene substrates.
Typically such adhesive blends include the grafted propylene copolymer blended with a propylene homopolymer or an ethylene/propylene copolymer and, optionally, a third component such as a linear low density polyethylene (LLDPE), a hydrocarbon rubber such as ethylene-propylene-diene monomer (EPDM) rubber or ethylene-propylene rubber (EPR), a poly(1-olefin) such as poly(butene-1), or an ethylene polymer such as high molecular weight low density polyethylene (HMW LDPE).
Prior polypropylene-based graft copolymers have been limited in terms of their maximum attainable functionality, i.e., the concentration of grafted acid or derivative, and/or have exhibited unacceptably low viscosity, i.e., high melt flow rates (MFRs). High acid or acid derivative functionalities are desirable in order to obtain acceptable levels of adhesion at desirably low graft copolymer concentrations with the adhesive blends. Furthermore, high MFR propylene copolymers grafts are difficult to process and, if the MFR is sufficiently high, will adversely affect the processing characteristics of the resulting adhesive blend and its adhesion properties.
Due to the inherently difficult process of grafting ethylenically unsaturated acids or acid derivatives to propylene polymers, prior attempts to graft propylene polymers by high temperature thermal grafting techniques in extruders has resulted in maximum grafted acid or derivative concentrations of no more than about 1 to about 1.2 wt. %. Using peroxide or other free radical generating catalysts in the extruder/reactor makes it possible to achieve higher graft contents but results in chain scission, often referred to as xe2x80x9cvisbreaking,xe2x80x9d and the production of relatively low molecular weight grafted and ungrafted fragments. The presence of these low molecular weight species produces a corresponding undesirable increase in MFR. While the low molecular weight species can be removed from the grafted reaction product, such as by solvent extraction, such procedures are costly and they result in loss of desired functionality.
The aforementioned problems are well recognized and described in the prior art. For example, U.S. Pat. No. 5,367,022 points out that when polypropylene homopolymer is grafted with maleic anhydride using peroxide to a target grafted anhydride concentration of 2% by weight, the resulting reaction product exiting the extruder reactor contains about 1.5 to 2.2 wt. % grafted monomer. After refining by solvent extraction to remove low molecular weight polymer fragments, the product only contains about 0.6 to about 1.2 wt. % grafted monomer. The reference also states that grafting graft polypropylene backbones with acid or derivative monomers resulted in unacceptably large increases in the melt flow rate of the product as compared to that of the ungrafted polypropylene, principally due to chain scission. It goes on to indicate that polypropylene homopolymer backbones having an initial melt flow rate (MFR) of about 2 to about 5 g/10 min before grafting typically exhibit MFR values on the order of about 1500 g/10 min after grafting with 1wt. % maleic anhydride and concludes that such MFRs are far too high for economical pelletizing operations.
While it is an object of U.S. Pat. No. 5,367,022 to provide propylene polymers and adhesive blends based thereon having high grafting monomer functionality and relatively low MFRs, the maximum level of maleic anhydride grafted onto the impact copolymer was 2 wt. %. Moreover, at that graft level the MFR of the grafted product was 398 g/10 min, a level generally considered to be unacceptable for commercial processes. In commercial processing operations for adhesive applications it is generally preferred that the grafted product have a MFR less than 300 g/10 min and, more preferably, 250 g/10 min or lower. It is further observed in the examples of the patent that efforts to increase the amount of maleic grafted anhydride by increasing the amount of peroxide catalyst used, had the opposite effect. Not only was the amount of maleic anhydride grafted reduced, the MFR of the product was increased to even higher and more unacceptable levels.
Other references report the use of various coagents in efforts to increase the amount of graft monomer reacted to propylene polymers and increase graft efficiency during the grafting process. Such processes are disclosed in U.S. Pat. Nos. 5,344,886 and 5,344,888. While graft monomer (maleic anhydride) levels as high as 3.47 wt. % are reported using vinyl acetate as a coagent, there is no mention of the MFR of the resulting functionalized products. Furthermore, the reactive nature of the coagents being used virtually assures that all or at least a substantial portion of the coagent is also reacted with the propylene polymer so that the resulting product would have a significantly different structure than functionalized products produced using the graft monomer by itself.
It is an object of the present invention to produce functionalized propylene polymer products having higher amounts of acid or acid derivative grafted, i.e., reacted to the polymer backbone. It is a further objective to obtain grafted propylene polymers having higher graft contents while maintaining relatively low MFRs. A still further object is to provide improved adhesive blends formulated with the grafted propylene polymers.
These and other objectives are achieved with the present invention wherein it has unexpectedly been found that a select group of propylene impact copolymers can be grafted with olefinically unsaturated carboxylic acid and derivative monomers to produce functionalized products characterized by high grafting monomer functionality and relatively low MFR.
Specifically, the improved grafted propylene copolymers of the invention are grafted impact copolymers comprising a propylene-ethylene impact copolymer characterized by having a relatively narrow molecular weight distribution and comprising a reactor-made intimate mixture of propylene homopolymer and 20 wt. % or more ethylene-propylene copolymer and having at least 1 wt. % grafting monomer selected from the group consisting of ethylenically unsaturated carboxylic acid or acid derivative grafted thereto. Preferably, said grafted impact copolymer having a graft to melt flow rate ratio of 1 or more.
Especially useful products are obtained using maleic anhydride as the graft monomer. Highly useful grafted products have MWDs of 7 or less and, more preferably, 6 or less. Rubber, i.e., ethylene-propylene copolymer, contents are most advantageously 25 wt. % or greater. The grafted products preferably have MFRs less than 300 g/10 min even at graft levels of 2 to 3 wt. %. The ability to achieve graft to melt flow rate ratios of 1 and above at high graft levels is highly advantageous and unexpected.
Adhesive blends comprised of 0.1 to 25 wt. % of the grafted impact copolymer and up to 99.9 ungrafted polyolefin blending resin are also disclosed. Highly useful adhesive blends contain up to 40 wt. % elastomeric polyolefin. In an especially preferred embodiment of the invention, the adhesive blends contain 0.25 to 15 wt. % grafted impact copolymer and 10 to 30 wt. % ungrafted elastomeric polyolefin with the balance of the composition being an ungrafted propylene polymer.
The present invention provides grafted, i.e., functionalized, propylene impact copolymer products characterized by having high graft monomer functionality and relatively low MFR. The products of these invention are produced using conventional peroxide-induced melt grafting procedures and without the use of coagents. Specific types of propylene impact copolymers, which are described in more detail below, are necessarily employed to achieve the improved results.
The grafted products of the invention may be conveniently prepared by melt blending the ungrafted propylene impact copolymers in the substantial absence of a solvent with a free radical generating catalyst, such as a peroxide catalyst, in the presence of the grafting monomer in a shear-imparting reactor, such as an extruder/reactor. Twin screw extruder/reactors such as those marketed by Coperion (formerly Werner-Pfleiderer) under the designations ZSK-53 and ZSK-83 are commonly used.
In one embodiment, peroxide catalyst is introduced to the molten polymeric composition followed by introduction of the grafting monomer. The grafting monomer may be introduced at a slightly higher temperature. The grafting reaction is carried at a temperature selected to minimize or avoid rapid vaporization and consequent losses of the catalyst and monomer. The monomer is typically about 1 to about 5 wt. % based on the total reaction mixture weight.
A temperature profile where the temperature of the polymer melt increases gradually through the length of the extruder/reactor up to a maximum in the grafting reaction zone, and then decreases toward the reactor exit is preferred. Temperature attenuation is desirable for product pelletizing purposes. The maximum temperature within the reactor should be such that significant vaporization losses and/or premature decomposition of the peroxide catalyst are avoided. For example, with di-t-butyl peroxide and 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane, maximum temperatures within the reactor should be maintained at or below about 220xc2x0 C. In contrast, the so-called xe2x80x9cthermalxe2x80x9d grafting processes of the prior ail which do not include catalysts, may use temperatures up to about 380xc2x0 C. The maximum useful temperature varies with the selection of catalyst. Examples of useful peroxide catalysts include: 1,1-bis(tert-butylperoxy)cyclohexane; n-butyl-4,4-bis(tert-butylperoxyvalerate); 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane; 2,2-bis(tert-butylperoxy)butane; dicumylperoxide; tert-butylcumylperoxide; xcex1xcex1xe2x80x2-bis(tert-butylperoxypreoxy-isopropyl)benzene; di-tert-butylperoxide (DTBP); 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane; and the like. Since substantial amounts of solvent are to be avoided, the catalyst and monomer are preferably added in neat form to the reactor. Additional details regarding the grafting procedure and reactor extruder are provided in U.S. Pat. No. 5,367,022 which is incorporated herein by reference.
The grafting monomer used is at least one polymerizable, ethylenically unsaturated carboxylic acid or acid derivative, such as an acid anhydride, ester, salt or the like. Such monomers include but are not necessarily limited to the following: acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, maleic anhydride, 4-methyl cyclohex-4-ene-1,2-dicarboxylic acid anhydride, bicyclo(2.2.2)oct-5-ene-2,3-dicarboxylic acid anhydride, 1,2,3,4,5,8,9,10-octahydronaphthalene-2,3-dicarboxylic acid anhydride, 2-oxa-1,3-diketospiro(4.4)non-7-ene, bicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride, maleopimaric acid, tetrahydrophthalic anhydride, norborn-5-ene-2,3-dicarboxylic acid anhydride, Nadic anhydride, methyl Nadic anhydride, Himic anhydride, methyl Himic anhydride, and x-methylbicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride(XMNA). Maleic anhydride is a preferred grafting monomer.
The improved functionalized products of the invention having a highly desired balance of high graft monomer content at relatively low MFR are obtained using specific propylene-ethylene impact copolymers, namely, impact copolymers of propylene and ethylene produced using gas-phase, stirred-bed polymerization processes. They are reactor-made intimate mixtures of propylene homopolymer and propylene-ethylene copolymer. More specifically they are produced in two reactors connected in series using high activity supported transition metal catalysts. Propylene homopolymer is produced in the first reactor and then introduced to the second reactor where additional propylene, ethylene, hydrogen and catalyst, as necessary, are metered to produce the intimate physical mixtures which comprise the propylene-ethylene impact copolymers utilized for the invention. Gas phase polymerizations of this type are described in the article by Ross, et al., entitled xe2x80x9cAn Improved Gas-Phase Polypropylene Processxe2x80x9d in Ind. Ene. Chem. Prod. Res. Dev. 1985, 24, 149-154, which is incorporated herein by reference.
Propylene-ethylene impact copolymers produced in gas-phase polymerizations of the above types are comprised of crystalline (propylene homopolymer) and amorphous or rubber (ethylene-propylene copolymer) phases.
It has unexpectedly been discovered that by grafting propylene-ethylene impact copolymers having high rubber contents and narrow molecular weight distributions (MWDs) it is possible to obtain grafted copolymers having significantly higher graft contents and lower MFRs than heretofore possible using peroxide-induced melt grafting procedures. To achieve these improved results, propylene-ethylene impact copolymers having rubber contents of 20% or greater and MWDs of 7 or below are employed. It is even more preferable when propylene-ethylene impact copolymers with rubber contents of 25% or more and MWDs of 6 or below are grafted. Especially useful grafted products are obtained when impact copolymers of these types are grafted with maleic anhydride.
While grafted products having lower graft levels and graft to MFR ratios can also be produced using the above-identified impact copolymers, the primary advantage is the ability to consistently and easily produce grafted products having higher functionality but with MFRs heretofore generally associated with grafted materials having much lower values. This, of course, makes it possible for the end user to use less of the functionalized material in adhesive blends resulting in considerable economic benefit. In general, the products of the invention will have at least 1 wt. % of the acid or acid derivative grafted. More typically the graft monomer will constitute 1.75 to 3.5 wt. % and, more preferably, 2 to 3 wt. % of the grafted impact copolymer.
By using the higher rubber content, narrower MWD propylene-ethylene copolymers, it is possible to consistently obtain graft to MFR ratios of 1 or more products having greater than 2 wt. % ethylenically unsaturated carboxylic acid or derivative grafted. The graft to MFR ratio, abbreviated G/M, is determined using the formula:
G/M=(weight percent monomer grafted÷MFR)xc3x97100
For example, a grafted product having 2 wt. % maleic anhydride grafted and an MFR of 20 would have a G/M ratio of 10. As used herein, the terms graft, grafted and grafting denote covalent bonding of the monomer to the propylene-ethylene copolymer chain. MFRs referred to herein are determined by extrusion plastometer in accordance with ASTM D 1238.
Whereas with prior art products it has been possible to obtain G/M ratios greater than 1 at graft levels less than 1.5 wt. %, G/M ratios typically decreased to fractional values as the graft content was increased. For example, with the impact copolymer grafts reported in U.S. Pat. No. 5,367,022 (Products I, J, K, L and M) graft levels of 1.37 to 2 wt. % are reported. However, only the lowest graft content product, Product M with a maleic anhydride content of 1.37 wt. % had a G/M ratio greater than 1. These grafted products were prepared using an impact copolymer having an ethylene content of 12-15 weight percent. This impact copolymer (Quantum PP-1510) upon analysis was shown to have a rubber content of only 17 wt. % and MWD of 9.3. As graft contents of the reference products were increased, there was a corresponding increase in MFR so that G/M ratios dropped to 0.89 and below. The highest graft content product, Product J with a maleic anhydride content of 2 wt. %, had a G/M ratio of only 0.5. In contrast, a typical product of the invention having a maleic anhydride content of about 2 wt. % has a G/M ratio of about 2.5. With the products of the invention, even at maleic anhydride contents of approximately 3 wt. %, G/M ratios greater than 1 are still obtained. This clearly demonstrates the unexpected improvement achieved with the products of the invention, i.e., the ability to achieve high graft contents at relatively low MFRs.
The improved products of the invention are useful for bonding propylene polymers to a variety of substrates including polyesters, nylon, ethylene-vinyl alcohol copolymers, metals and the like. They are typically employed in adhesive blends comprising the grafted propylene-ethylene impact copolymers of the invention with one or more ungrafted polymers most typically, but not necessarily limited to, polyolefins. The adhesive blend can contain 0.1 to 25 wt. % of the grafted product. More preferably, the grafted impact copolymer will comprise 0.25 to 15 wt. % of the adhesive blend. The balance of the adhesive blend comprises the ungrafted polyolefin product(s). The concentration of the graft monomner, i.e., ethylenically unsaturated carboxylic acid or derivative, in the adhesive blend is preferably in the range of about 0.01 to about 1 wt. %.
Thus, the adhesive blend may be characterized as comprising about 0.1 wt. % to about 25 wt. % of the grafted propylene-ethylene impact copolymer, up to 99.9 wt. % of an ungrafted polyolefin blending resin, a portion of which can be an ungrafted elastomeric polyolefin. The adhesive blends are preferably prepared by melt compounding the components with the usual additives (e.g., antioxidants, etc.) and pelletizing the blended material.
Ungrafted polyolefin blending resins may be ungrafted propylene polymers, preferably selected from propylene homopolymers, statistical copolymers of propylene and up to about 30 wt. % of ethylene or a C4-C6 1-olefin comonomer, or impact or xe2x80x9cmodified impactxe2x80x9d (sometimes referred to in the art as xe2x80x9csuper impactxe2x80x9d) propylene-ethylene copolymers. Impact copolymers useful as blending resins in the adhesive blends of the invention include the so-called TPOs or TPOEs. Modified impact copolymers useful as blending resins in the adhesive blends of the invention comprise blends of an impact copolymer and an ethylene polymer, and preferably comprise a blend of between about 95 wt. % and about 70 wt. % of (A) an impact copolymer selected from the group consisting of (a) reactor-made intimate mixtures of polypropylene and statistically oriented copolymers of propylene and ethylene, and (b) blends of polypropylene and statistically oriented copolymers of propylene and ethylene, and between about 5 wt. % and about 30 wt. % of (B) an ethylene polymer. The ethylene polymer is preferably selected from the group consisting of ethylene homopolymers, low density polyethylene (LDPE), ethylene-acid copolymers (e.g., ethylene-acrylic acid copolymer), ethylene-vinyl acetate copolymers, ethylene-acrylate copolymers, such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE), so-called very low density polyethylene (VLDPE), so-called ultra low density polyethylene (ULDPE), and blends thereof. Visbroken modified impact polymeric blends and means for preparing them are described in detail in Ross U.S. Pat. No. 4,375,531 (Mar. 1, 1983), the disclosure of which is incorporated herein by reference.
A portion of the ungrafted polyolefin used for the adhesive blend can be an elastomeric polyolefin. The adhesive blends can contain up to about 40 wt. % (based on total blend) elastomeric polyolefins such as ethylene/propylene rubber (EPR) or ethylene-propylene-diene monomer (EPDM) rubber. Especially useful adhesive blends contain 10 to 30 wt. % elastomeric polyolefin.
The grafted products of the invention may also be used as compatibilizing agents when blending two or more dissimilar resins. The grafted products of the invention may also be used as coupling agents for filled polyolefins such as polypropylene filled with glass fibers, mineral fillers, wood, flour and the like.